The Phenomenon of Polarization in an X-ray Tube

S. Ratner, “Polarisation Phenomena in an X-ray bulb.” Nature, 107, p. 522, 1921.

The voltage necessary to obtain a discharge in an X-ray tube has hitherto been considered dependent on the gas pressure in the tube (other conditions being equal).

Ratner’s experiments show that the matter is (apparently) more complicated, and that the “hardness” of the tube cannot be explained solely by a decrease in the gas pressure.

A current was passed for several days through a tube connected to a Gaede pump and a McLeod manometer. Despite the constant pressure, the tube became harder and harder, and finally the current ceased altogether.

In order to obtain a discharge again, it was necessary either to raise the voltage or to increase the gas pressure in the tube. The author compares this phenomenon with the polarization of cells. Such “polarization” could proceed so far that, for example, raising the voltage to 50,000 volts, despite a comparatively high pressure of 0.060 mm, could not again produce a discharge.

If the tube was allowed to “rest,” it returned to its initial state, but even a brief passage of current again brought about the phenomenon of “polarization.” That the effect does not depend on a change in the properties of the gas in the tube under the influence of the discharge was established by passing current through a lateral, wide tube attached to the X-ray tube. In it the electrodes were identical and at the same distance from one another as in the main tube; when, after prolonged action, the main tube had been “polarized” so that the greatest applied voltage no longer produced a discharge (pressure = 0.050 mm), a potential difference of 1200 V applied to the lateral tube produced a completely normal discharge in it.

The phenomenon described can apparently be attributed to the destruction by the discharge of the gas layer on the surface of the electrodes of the X-ray tube; and the commonly observed increase in the hardness of the tube during prolonged operation is caused not only by absorption of the residual gas.

A. Trapeznikov.